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Light-emitting diode

Light emitting diodes (LED) are much more efficient at producing visible light than traditional light sources such as filament bulbs, and fluorescent lights. LED are made of semi-conductor material; light is produced when electrons are passed through the semiconductor material. Unlike traditional light sources LED do not get hot, so very little energy is wasted as heat. LED also do not produce ultraviolet or infrared rays which are light waves which can not be seen by the human eye. What LED do is convert a higher percentage of energy into light within the visible spectrum which means that less power is required to produce the same amount of useful light.

The lifetime of an LED lamp is around 50,000 hours; that means that under normal use an LED bulb will not need replacing for 12 years. Using LED lamps means that replacement bulb costs are reduced, as well as man hours spent replacing the lamps. Most LED lamps consume between 1 and 3 Watts of energy. This means that the electricity cost to run an LED bulb is much lower than standard incandescent, compact fluorescent and halogen lamps. In some cases switching to LED lamps can yield a 95.4% saving on electricity charges.

Some materials used in the manufacture of other lighting sources, such as fluorescent lamps, require the use of harmful chemicals. Fluorescent lights contain mercury, which means that when the lamp comes to the end of its life it has to be disposed of in a landfill site. LED lamps contain no toxic materials which mean when they come to the end of their life they can be recycled, a better solution for the environment.

In the present day everybody should be responsible for helping to reduce carbon emissions and halt global climate change. In the UK over 50% of household carbon emissions are attributable to lighting. Changing to LED bulbs would make a very significant difference to the amount of energy consumed by UK homes and businesses.


Efficiency and operational parameters

One of the key advantages of LED-based lighting is its high efficiency, as measured by its light output per unit power input. White LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with a luminous efficacy of 18-22 lumens per watt [lm/W]. For comparison, a conventional 60-100 W incandescent lightbulb produces around 15 lm/W, and standard fluorescent lights produce up to 100 lm/W. A recurring problem is that efficiency will fall dramatically for increased current. This effect is known as droop and effectively limits the light output of a given LED, increasing heating more than light output for increased current.

In September 2003, a new type of blue LED was demonstrated by the company Cree, Inc. to provide 24 mW at 20 milliamperes [mA]. This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006 they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA.

Also, Seoul Semiconductor has plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be approaching an order of magnitude improvement over standard incandescents and better even than standard fluorescents. Nichia Corporation has developed a white LED with luminous efficiency of 150 lm/W at a forward current of 20 mA.

Cree issued a press release on November 19, 2008 about a laboratory prototype LED achieving 161 lumens/watt at room temperature. The total output was 173 lumens, and the correlated color temperature was reported to be 4689 K.


Lifetime and failure

Solid state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Many of the LEDs produced in the 1970s and 1980s are still in service today. Typical lifetimes quoted are 25,000 to 100,000 hours but heat and current settings can extend or shorten this time significantly.


Advantages

  • Efficiency : LEDs produce more light per watt than incandescent bulbs
  • Color : LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
  • Size : LEDs can be very small (smaller than 2 mm2) and are easily populated onto printed circuit boards.
  • On/Off time : LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds.LEDs used in communications devices can have even faster response times.
  • Cycling : LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
  • Dimming : LEDs can very easily be dimmed either by Pulse-width modulation or lowering the forward current.
  • Cool light : In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
  • Slow failure : LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
  • Lifetime : LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000-2,000 hours.
  • Shock resistance : LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
  • Focus : The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
  • Toxicity : LEDs do not contain mercury, unlike fluorescent lamps.

  • Sustainable Lighting

    Efficient lighting is needed for sustainable architecture. A 13 watt LED lamp produces 450 to 650 lumens. which is equivalent to a standard 40 watt incandescent bulb. A standard 40 W incandescent bulb has an expected lifespan of 1,000 hours while an LED can continue to operate with reduced efficiency for more than 50,000 hours, 50 times longer than the incandescent bulb.


    An Environmentally Friendly Option

    A single kilowatt-hour of electricity will generate 1.34 pounds (610 g) of CO2 emissions. Assuming the average light bulb is on for 10 hours a day, a single 40-watt incandescent bulb will generate 196 pounds (89 kg) of CO2 every year. The 13-watt LED equivalent will only be responsible for 63 pounds (29 kg) of CO2 over the same time span. A building's carbon footprint from lighting can be reduced by 68% by exchanging all incandescent bulbs for new LEDs.

    LEDs are also non-toxic unlike the more popular energy efficient bulb option: the compact florescent a.k.a. CFL which contains traces of harmful mercury. While the amount of mercury in a CFL is small, introducing less into the environment is preferable.


    Economically Sustainable

    LED light bulbs could be a cost effective option for lighting a home or office space because of their very long lifetimes, even though they have a much higher purchase price. The high initial cost of the commercial LED bulb is due to the expensive sapphire substrate which is key to the production process. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted.